Titanium forging is a manufacturing process involving the shaping of titanium metal using localized compressive forces and heat. Major quality problems may occur during titanium forging due to abnormal grain growth (AGG) within forgings after a final heat treatment. This problem may be observed with any beta annealed forging. No prior art method exists to salvage such parts past the billetizing process.
Beta annealed forgings experience this problem because of significant cumulative reduction in area, from the ingot to billet and from the billet to the finished working, creating a significant amount of localized plastic deformation within the forging volume. For applications requiring beta annealing to achieve enhanced damage tolerance and fracture toughness, Ti-6Al-4V forgings are often heated to above the beta transus of the material and held for sufficient time to transform the alloy microstructure within the forging from a mixture of alpha and beta phases into a singular phase (beta) phase at solution heat treatment temperature. The forging is subsequently cooled at prescribed cooling rates, for example, rates equivalent to air cooling for beta annealing, or faster cooling rates such as that of water quenching for beta solution treated application. Upon cooling to room temperature, the forging can then be stabilized, or alternatively aged or over-aged to achieve a final temper. In this case, the final temper is beta anneal, which includes stabilization to achieve a good combination of strength and damage tolerance.
When too much cumulative hot work is performed during the reduction of the alloy from ingot to the final shape within the alpha-beta forging temperatures, the grains can become highly deformed, and the amount of deformation might not be uniformly distributed within the forging. The uneven strain distribution and the high levels of stored strain energy within the forging, upon subsequent beta annealing heat treatment and shortly after the temperature exceeds the beta transus, can result in accelerated grain growth at the zones with the highest level of plastic strain at the expense of a more copious, and uniform nucleation of new grains. Forgings with microstructures consisting of non-uniform transformed grain size can greatly degrade mechanical properties and durability of the resultant forgings, with fatigue crack initiation and possibly propagation being the most affected.
FIG. 1 shows a typical prior art beta annealed Ti-6Al-4V forged macrostructure exhibiting both fine grains at the periphery and very coarse grains in the middle of the forged billet. The forging pictured had undergone more than 6×-8× reduction prior to beta anneal. The excessive plastic deformation resulted in significant driving force for grain growth in the middle of the forging.